Sensors that are fiber optically based are becoming increasingly common as accurate high-speed indicators of a wide variety of environmental conditions. Embedded fiber optic sensors are already finding use in the aerospace industry's push to build "smart structure" aircraft that fly-by-light. Due to benefits in power consumption, size, weight and immunity to electromagnetic interference, embedded fiber sensors will find use in the next generation of utility plants, skyscrapers and bridges. Potential advantages of the instant invention include: 1) a sensor that can monitor any structure during the entire manufacturing cycle with continuous monitoring of the structure's integrity at any localized section without a destructive effect on the sensor; 2) a sensor that can monitor a structure's deformation during its lifetime; and 3) a sensor that can monitor perforation, low velocity impact, fatigue, and overload damage to a structure.
Prior inventions that have taught of using optical fibers for damage sensing include: 1) U.S. Pat. No. 5,015,842 where the optical fibers are placed in a pattern of a structure so that when an optical fiber is fractured, a zone of structural damage is detected; and 2) U.S. Pat. No. 4,936,649 which teaches of a damage sensor detector which again uses optical fiber fracture to detects localized damage within a structure prone to cracking or delamination at known loads. Both of these sensors have the disadvantage of being destroyed once structural failure occurs. To overcome this problem, the present invention is a non-destructive type sensor.
The present invention use unobvious analytical design techniques using linear work-hardening elastic-plastic metal coatings which can permanently deform in response to a preset damage load. The basic concept of using a fiber optic damage detection device was first mentioned in Dasgupta, A., Sirkis, J. S., and Liu, C., "The importance of Coating to Structurally Embedded Optical Fiber Sensors," Proceedings of ISA International Conference, New Orleans, pp. 1673-1693, 1990. This article discusses the design of linear elastic metal coatings which minimize concentrations of transverse stress components in the host material and sensor that are induced by structurally embedded optical fibers. However, this teaching does not disclose the instant device's critical geometrical and material design features required for durable, reliable and continuous use under adverse environmental conditions.